Industrial fabrics means an endless structure in the form of a continuous loop such as one used as a forming, press or dryer fabric (paper machine clothing or PMC) as well as a process belt such as a shoe press, calendar, or transfer belt used on a paper machine. Industrial fabrics also means a fabric used in textile finishing processes. Industrial fabrics also include other endless belts where a high degree of compressibility and resiliency is required.
While the discussion herein concerns for the most part the papermaking process in general, the application of the invention is not considered limited thereto.
In this regard, during the papermaking process, for example, a cellulosic fibrous web is formed by depositing a fibrous slurry, that is, an aqueous dispersion of cellulose fibers, onto a moving forming fabric in a forming section of a paper machine. A large amount of water is drained from the slurry through the forming fabric, leaving the cellulosic fibrous web on the surface of the forming fabric.
The newly formed cellulosic fibrous web proceeds from the forming section to a press section, which includes a series of press nips. The cellulosic fibrous web passes through the press nips supported by a press fabric, or, as is often the case, between two such press fabrics. In the press nips, the cellulosic fibrous web is subjected to compressive forces which squeeze water therefrom, and which adhere the cellulosic fibers in the web to one another to turn the cellulosic fibrous web into a paper sheet. The water is accepted by the press fabric or fabrics and, ideally, does not return to the paper sheet.
The paper sheet finally proceeds to a dryer section, which includes at least one series of rotatable dryer drums or cylinders, which are internally heated by steam. The newly formed paper sheet is directed in a serpentine path sequentially around each in the series of drums by a dryer fabric, which holds the paper sheet closely against the surfaces of the drums. The heated drums reduce the water content of the paper sheet to a desirable level through evaporation.
It should be appreciated that the forming, press and dryer fabrics all take the form of endless loops on the paper machine and function in the manner of conveyors. It should further be appreciated that paper manufacture is a continuous process which proceeds at considerable speeds. That is to say, the fibrous slurry is continuously deposited onto the forming fabric in the forming section, while a newly manufactured paper sheet is continuously wound onto rolls after it exits from the dryer section.
Base fabrics, which form an important portion of the above discussed fabrics, take many different forms. For example, they may be woven either endless or flat and subsequently rendered into endless form with a woven seam using one or more layers of machine direction (MD) and cross machine direction (CD) yarns. Also such fabrics may employ what is referred to as a pin seam also formed from MD yarns to allow installation on the paper machine. Further, the base fabrics may be laminated by placing one base fabric within the endless loop formed by another base fabric, and joining or laminating them together by various means known to those skilled in the art such as by needling staple fiber batt through both base fabrics to join them to one another.
In paper machine clothing (PMC) especially press fabrics used in the press section of a paper machine, the fabric has one or more “base structures” formed from yarns and staple fiber batt needled into usually at least the sheet contact surface. The press fabric has an initial thickness, mass, and consequent void volume (the calculated volume based upon this mass and thickness) which equates to water handling capacity. They also have a measurable contact area.
Since press fabrics are subjected to normal loads (normal to the fabric plane in use) as it passes through one or more press nips, the fabric has a compressed void volume and surface contact area as well. While there have been various attempts to change the degree of compressibility, press fabrics become progressively thinner over time and millions of nip cycles. Eventually they must be removed due to various reasons such as lack of water handling capability, marking, or press vibration. When they have reached the end of their useful lifetime they must be removed and replaced with a new fabric.
New fabrics also go through a break in period wherein the density is not ideal and water handling is less than optimum. Accordingly, an ideal press fabric is one that has near constant or steady state performance (for example water handling capability) from day one until it is removed from the paper machine.
Various attempts have been made to affect press fabric properties, especially compressibility and resiliency. One attempt has been to introduce “elastic” yarns into structures. One example of this is seen in PCT application WO 2004/072368 A1. There are shortcomings to this approach however. The compressibility is only due to the elastic portion (in the through thickness direction) of the yarn, and is therefore limited to such. While larger yarns can be used, there is eventually a diminishing return on performance. Also large yarns are heavy, and can cause objectionable sheet marking. If the yarn is a sheath/core type, there is always the danger of delamination of the sheath from the core. Finally, the degree of compressibility is limited to a maximum of some fraction of the yarn diameter.
Another example is U.S. Patent application 2007/0163741 A1 which incorporates an array of compressible sheath/core yarns attached to the backside of a seamed press fabric. It is taught that the sheath is elastomeric, and can provide vibration dampening effects. It further teaches that the yarn core alone can be 200 to 2000 denier, and a total size of 0.30 to 1.2 mm in diameter. Such yarn sizes can be limited in use due to weight and potential marking considerations.
A further example is taught in U.S. Pat. No. 4,350,731 which teaches the use of wrapped yarns to make a compressible press fabric structure. Again the degree of compressibility and recovery is due to only the elastomeric wrapping sheath layers.
Another example of this type of structure is taught in GB 2 197 886. This patent discloses compressible yarns which alternated in some manner with functional (tensile) load bearing yarns to provide, under an applied normal load, a dense, quasi-single layer base structure without “knuckles” and with long weave floats to provide a quasi-crossless base construction.
Yet another example is disclosed in U.S. Pat. No. 5,087,327 to Hood, which relates to a composite yarn for use in a papermaker's fabric. The composite yarn includes a soluble core surrounded by a layer of non-soluble monofilament.
Yet another example is disclosed in U.S. Pat. No. 5,597,450, which relates to a paper machine dryer fabric including hollow thermoplastic monofilaments in the cross-machine direction.
A further example is disclosed in U.S. Publication No. 2002/0100572, which relates to a papermaking fabric having beam construction yarns that resist lateral, vertical and torsional deflection as well as compression when interwoven. The yarns are non-circular in cross-section, such as I-beam, H-beam, and box-beam.
A further example is a structure taught in U.S. Pat. No. 4,781,967. Such a structure is defined to be relatively incompressible as the stacked yarn arrays do not compress nor move relative to any other layer. In other words, when there is an applied load normal to the plane of the structure, there is little thickness change, except for any yarn deformation which is permanent. If an elastomeric (in the yarn thickness direction) is employed as the yarns in an entire layer, the compressibility of the structure is limited to some portion of that yarn diameter.
Yet another example is taught in U.S. Pat. No. 4,555,440. Again this structure is considered incompressible as there is little through thickness change when a normal load is applied or removed.
Incorporating “elastic” (in the thickness or radial direction) yarns into fabrics has affected to some degree the resiliency or spring back of these fabric structures once the normal load is removed. But again, using these yarns, the compressibility and spring back is limited to some portion of the yarn diameter at most.
As stated above, because of this limited resiliency, press fabrics have a relatively high void volume to handle water when new, more than is ideally required. They will compact and reach an optimum performance level for a period of time. However as they have limited resiliency, they will continue to compact, eventually requiring removal and replacement.